Fix ImportError in preliminary analytics support

A customer is trying to exercise the uncommitted, experimental analytics support provided in -PYCBC-370-. They are seeing this:

Looking at that part of the code:

def _analytics_query(self, query, host):

    from *cbas* import AnalyticsRequest, AnalyticsQuery

    if not isinstance(query, AnalyticsQuery):

        query = AnalyticsQuery(query)

    return AnalyticsRequest(query, host, self)

I suspect that the cause of the ImportError is that this should be 'couchbase.cbas' rather that cbas. 'cbas' will work when running from the couchbase-python-client directory, but not elsewhere, whereas I think couchbase.cbas should work everywhere. Will test and confirm ASAP.